EXHAUST MANIFOLD REMOVAL PROCEDURE

Under hood, center, rear engine area, above valve cover, mounted in bracket.

From autozone.com:

Manifold Air Pressure Sensor

Fig. MAP sensor specifications-note how altitude and pressure affect the voltage readings
REMOVAL & INSTALLATION

1. Remove the intake manifold top cover.
2. Disconnect the crankcase breather tube from the throttle body.
3. Remove the MAP sensor from the intake manifold.
4. Detach the MAP sensor electrical connector.
5. Inspect the MAP sensor grommet for wear or damage and replace as needed.

To install:

1. Installation is the reversal of the removal procedure.

check out your procedure for this as it should not be necessary to remove any manifolds to replace plugs. Ask the service centre as to what tool you should have for the job or get a workshop or owner manual which will explain the way to remove the plugs.

The knock sensor is located under the intake manifold to replace do the following
Disconnect the negative battery cable.
Remove the intake manifold.
Disconnect the sensor connector.
Remove the sensor from the engine block.
Installation is the reverse of the removal procedure

It doesn't have to come off but it will make your life a hell of a lot easier to access the exhaust bolts if it does!!!!

Note, removing the upper intake manifold is not necessary when removing lower intake manifold, they are removed as an assembly.
Disconnect negative battery cable. Drain cooling system. Remove air cleaner assembly and inlet duct. Mark and remove distributor. Remove upper radiator hose at thermostat housing. Disconnect accelerator and cruise control cables. Remove heater hoses from intake manifold. Remove EGR pipe.
Disconnect fuel lines at rear of engine. Remove AC compressor and set aside(leave hoses connected). Remove canister purge solenoid. Disconnect theSCPI injector, ignition coil, Ignition Control Module, generator,IAC motor, TP sensor, O2 sensor, Exhaust Gas Recirculation valve and Engine Coolant Temperature sensor wiring harness connectors. Remove PCV valve and hose. Remove lower intake manifold bolts. Remove lower intake manifold.
Installation.
Always use New O-rings when reconnecting fuel lines at rear of engine. Apply a bead of RTV silicone sealer along the front and rear sealing surfaces of the block.See Fig.6. Install new gaskets and intake manifold on block while RTV is still wet to the touch. Use Threadlock 12345382 or equivalent to lower intake manifold bolts. Tighten lower intake manifold bolts in sequence to specification table. See fig. 7. To complete installation, reverse removal procedure.

3.8L Engine

NOTE The 3.8L (VIN K) non-supercharged engine uses a two-piece intake manifold consisting of an upper air plenum which mounts the throttle body and a lower intake manifold assembly which houses the fuel injectors. The 3.8L (VIN 1) supercharged engine uses a similar lower intake manifold. The supercharger serves as the upper manifold.
Lower Manifold

NOTE Two bolts which fasten the lower intake manifold to the cylinder head are accessible only after the upper intake manifold is removed. These bolts are located in the right front and left rear corners of the lower intake manifold. Remove the upper intake manifold to service the lower intake.
The 3.8L (VIN 1) supercharged engine uses a manifold similar to the 3.8L (VIN K), but the supercharger serves as the upper manifold.

1. Disconnect the negative battery cable.
2. On 3.8L (VIN 1) engines, remove the supercharger assembly. Please see the procedure in this section.
3. Remove the upper intake manifold using the procedure above.
4. Remove the EGR outlet pipe from the intake manifold.
5. Detach the engine coolant temperature sensor wiring harness from the sensor.
6. Remove the lower intake manifold bolts and carefully lift off the manifold.

To install:


Fig. Lower intake manifold bolt torque sequence-3.8L (VIN K) engine

1. Clean all parts well with degreaser, especially the gasket seal surfaces on the upper and lower manifold pieces and between the lower manifold and the cylinder heads..
2. Install new gaskets to the cylinder heads and new seals to the engine block. Carefully lower the manifold into place. Apply thread-locking compound to the bolt threads. Make sure to install the two hidden bolts in the lower intake manifold. Hand start all bolts, then torque evenly to 11 ft. lbs. (15 Nm). Follow the toque sequence, starting in the center, then working outwards in a circle.
3. Install the remaining components in the reverse order of the removal process.

Upper Intake Manifold (Plenum)

1. Disconnect the negative battery cable.
2. On 3.8L (VIN 1) engines, remove the supercharger assembly. Please see the procedure in this section.
3. Tag for identification, then remove the right side spark plug wires.
4. Disconnect the ignition wires from the fuel rail.
5. Remove the fuel rail. Please see Section 5.
6. Remove the cable bracket from the intake manifold.
7. Remove the throttle body.

NOTE Two bolts which fasten the lower intake manifold to the cylinder head are accessible are accessible only after the upper intake manifold is removed. These bolts are located in the right front and left rear corners of the lower intake manifold.

1. Remove the upper intake manifold bolts and carefully lift the upper intake manifold from the engine.

Fig. Upper intake manifold and bolt torque sequence-3.8L (VIN K) engine
To install:

1. Clean all parts well with degreaser, especially the gasket seal surfaces on the upper and lower manifold pieces.
2. Install new gaskets to the cylinder heads and new seals to the engine block. Carefully lower the manifold into place. Make sure to install the two hidden bolts in the lower intake manifold. Hand start all bolts, then torque evenly to 89 inch lbs. (10 Nm). Follow the torque sequence, starting in the center, then working outwards in a circle.
3. Install the remaining components in the reverse order of the removal process.

Removal & Installation

1. Disconnect the negative battery cable.
2. Remove the intake manifold.
3. Disconnect the sensor connector.
4. Remove the sensor from the engine block.
5. Installation is the reverse of the removal procedure.
   
   Fig. Knock sensor removal and installation-Mazda 3
   

1. Disconnect the negative battery cable.
2. Remove the intake manifold bracket.
3. Disconnect the sensor connector.
4. Remove the sensor from the engine block using special tool 49H018001 or equivalent.
5. Installation is the reverse of the removal procedure.
   
   GL G

If you have never replaced a head gasket before the best advice I can give is leave it to a professional, as on modern vehicles there are a number of electrical, emissions, and vacuum connections that can and will cause very serious problems if not disconnected or reconnected properly.

If you feel the need to replace it yourself it's essentially10 steps.

KEEP ALL FASTENERS, AND WIRING HARNESSES ORGANIZED

remove the upper fan shroud
remove the serpentine belt, fan and pulley
loosen and remove the alternator
remove the air cleaner assembly up to the throttle body
remove the upper radiator hose, thermostat, and housing loosen and remove the heater hose
remove all electrical connections from the upper intake, then remove the fuel lines from the lower intake. Good luck with the rear fuel connection as it is snug against the block and the firewall
loosen and remove the heater hose bracket on the intake manifold
remove the intake manifold
remove the remaining bolts from the accessory bracket on the front of the engine and set the A/C compressor and PS pump aside (DO NOT DISCONNECT HOSES)
remove the oil dipstick/filler tube bracket and pull it out
disconnect exhaust manifold from exhaust down pipe
remove exhaust manifold nuts, then remove exhaust manifold
compress and remove rocker arms and push rods (KEEP IN ORDER)
remove head bolts
Remove head without prying between head and block
Send head out for machining or replace with new head

Installation in reverse of removal

Hope this helps

REMOVAL PROCEDURE

1. Turn OFF the ignition.
2. Disconnect the EGR valve electrical connector.
3. Remove the bolt retaining the pipe assembly from the EGR valve and carefully pull the pipe assembly back.
4. Remove the EGR valve retaining bolts.
5. Remove EGR valve assembly.
6. Remove the gasket.

INSTALLATION PROCEDURE

1. Install the EGR valve with a new gasket to the intake manifold. NOTE: Refer to Fastener Notice in Service Precautions.
2. Reinstall the bolts through the EGR valve into the intake manifold. Tighten Tighten the EGR bolts to 30 N.m (22 lb ft).
3. Connect the exhaust pipe assembly to the EGR valve.
4. Reinstall the exhaust pipe bolt. Tighten Tighten the nuts to 30 N.m (22 lb ft).
5. Connect the EGR valve electrical connector